As far as passive, two-dimensional scanning is concerned, especially in the visible region of the spectrum, present-day technology has provided a satisfactory answer in the form of various detector arrays and matrices, such as, e.g., CCD devices. The problem becomes more complex when the scene to be scanned requires active illumination. Some of the known devices use the direct scanning solution, in which the illuminator beam is passed through a scanning device and impinges upon the scene. The reflected light, after being steered by the same scanner, produces an image on a detector. The moving parts and the required synchronization constitute serious drawbacks.
At the other end of the spectrum of scanning devices is the detector array. The light source illuminates the entire scene, which is “seen” in full by an array of detectors.
The practical value of these solutions depends on various parameters, associated partly with the device itself and partly with the scene and the atmospheric conditions, in conjunction with the demands of the overall system. Sometimes a hybrid solution is indicated, in which a detector array having a partial field of view only, and an accordingly shaped illuminator beam, partially scan the scene. Such complex solutions should be selected when three-dimensional profiling imaging is required, especially in the infrared region of the spectrum, and more particularly when eye-safe illumination is to be used.
The heart of advanced, modern, non-eye-safe three-dimensional imagers is the high-powered laser, illuminating the scene or part of it, and an array of avalanche photodiodes (APDs) with the associated range-measuring and image-creating electronics. The most serious disadvantage of these devices is the demand for arrays of detectors that must be both sensitive and have a very rapid response to the short pulses of the laser illuminators.
It is these illuminators and the corresponding detectors that are used in military range finders. The light radiated is not eye-safe and the APD detectors with their range-imaging electronics are very complex and expensive, in comparison with the conventional detector arrays used in video cameras.
Lately, due to the growing demand for eye-safe devices and due to the latest developments and availability of solid-state, eye-safe laser illuminators, a similar demand is felt for appropriate APD detector arrays for eye-safe, three-dimensional profiling imaging. This means a low-cost array, or at least a sub-array, of APDs, sensitive to the wavelength of eye-safe laser light (1.5 to 1.6 μm). Until recently, no such array has been available. Detectors to be used with laser range finders operating at such a wavelength are too difficult to produce as multi-element arrays. Despite efforts reported in the literature, so far only partial success has been achieved in developing a prototype of such an APD having a quasi-array of a few elements, which suffers from enormous electronic complexity and is very expensive, making it impractical for world market demands.
U.S. Pat. No. 3,991,318 (Duguay) discloses an optical detection system utilizing an organ array of optical fibers, each cut to different lengths, one end of each of the fibers being arranged in a grid under a planar writing surface. Detection means are coupled to the other end of each of the fibers. The utilization of a detector for each of the fibers renders the system cumbersome and costly.